refactor(icons): centralize icon resolution into Paths.resolveIconPath/resolveIconUrl (#1880)

Supersedes #1878. Rather than duplicating the moddedAppId + file path
substitution pattern inline across 8 files, this introduces two
centralized functions in Paths.qml:

- resolveIconPath(iconName): for Quickshell.iconPath() callsites,
  with DesktopService.resolveIconPath() fallback
- resolveIconUrl(iconName): for image://icon/ URL callsites

All consumer files now use one-line calls. When no substitutions are
configured, moddedAppId() returns the original name unchanged (zero
cost), so this has no impact on users who don't use the feature.

Affected components:
- AppIconRenderer (8 lines → 1)
- NotificationCard, NotificationPopup, HistoryNotificationCard
- DockContextMenu, AppsDockContextMenu
- LauncherContent, LauncherTab (×3)

Co-authored-by: odtgit <odtgit@taliops.com>

quickshell/Modules/Dock/DockContextMenu.qml

@@ -329,7 +329,7 @@ PanelWindow {
 
                             IconImage {
                                 anchors.fill: parent
-                                source: modelData.icon ? Quickshell.iconPath(modelData.icon, true) : ""
+                                source: modelData.icon ? Paths.resolveIconPath(modelData.icon) : ""
                                 smooth: true
                                 asynchronous: true
                                 visible: status === Image.Ready